1. Field of the Invention
This invention relates generally to the field of scanning electron microscopes (SEM) and, more particularly, to an SEM apparatus and a method for scanning an electron beam across a topographical feature on a planar surface and delecting resulting secondary electrons which are collected by a pair of oppositely disposed electron detectors which are alternately switched on and off to produce first and second electric signals which are combined to produce a composite electric signal which provides an enhanced, symmetrical image of the junctures of the planar surface with the opposite lower edges of the topographical feature, which enhanced image can be used to determine very small widths of topographical features such as very narrow conductive coatings on a semiconductor substrate.
2. Description of the Prior Art
Goldstein et al have described the use of an Everhart-Thornley (E-T) scintillator-photomultiplier electron detector in scanning electron microscopy. An E-T detector is generally cylindrical in shape and contains a scintillator material which converts impinging electrons to photons which are conducted to a photomultiplier which produces an electric signal corresponding to the intensity of the collected electrons. When an electron beam strikes the surface of a specimen, both secondary electrons and backscattered (reflected) beam electrons escape the specimen. Secondary electrons are emitted with an average energy of 3-5 eV. In a typical SEM, the electron beam has an energy of, for example, 20 keV, and the backscattered beam electrons have a much higher energy than the secondary electrons which are emitted. Furthermore, the backscattered electrons are highly directional, and their directions are not affected by the positive bias voltage on the Faraday cage of an E-T detector. In fact, when only backscattered electrons are to be detected, the cage bias voltage is not required, as the E-T detector will collect all backscattered electrons whose take-off angles are within the range of the detector's solid angle of collection. However, when it is desired to collect secondary electrons, a positive bias potential up to as +300 V is placed on the Faraday cage. ("Scanning Electron Microscopy and X-ray Micro Analysis", Plenum, 1981, pages 146-151).
Jackman has described the use of a plurality of E-T detectors for detecting backscattered beam electrons. Through the use of optical switches, the signals from these detectors can be viewed individually to give asymmetric detection, summed to give the equivalent of a large symmetric detector, or subtracted (Jackman, J., June 19, 1980; Industrial Research and Deveopment, Vol. 22, page 115).
Saitou et al have described a digital computer-controlled electron beam microfabrication system for recording submircometer patterns by an electron resist on a semiconductor wafer. Registration of the beam is achieved by detecting backscattered electrons to locate the positions of registration or reference marks on the wafer. (Journal of Physics, Sect. E, Scientific Instruments, Vol. 7, Jan. 1974, pages 441-444).
Schur et al have described an SEM including a modified E-T scintillator-photomultiplier for detecting backscattered electrons in quantitative image analysis (stereology) of plane-polished sections. ("Improved Conditions for Back-Scattered Electron SEM Micrographs on Polished Sections Using A Modified Scintillator Detector", Conference, Scanning Electron Microscopy/1974, IIT Research Institute, Chicago, Ill., Apr. 8-11, 1974, pages 1,003-1,010).
Furukawa et al in U.S. Pat. No. 4,177,379 have described a backscattered electron detector for use in an electron microscope or electron beam exposure system, and including a scintillator for converting backscattered electrons to light, and a converter for converting the light to corresponding electric signals. FIGS. 5 and 6 show the use of the detector in an electron beam exposure system for locating a reference groove in a semiconductor substrate during the formation of a mask pattern using an electron beam resist.
Reimer in U.S. Pat. No. 4,308,457 has described an electron microscope which an electron beam impinges upon a specimen, and including a detector for separately detecting the secondary electrons released from a specimen and the secondary electrons produced by the beam electrons backscattered specimen.
Van Nieuwland et al in U.S. Pat. No. 3,920,990 have described an ion-scattering spectrometer including an ion beam source for directing an ion beam onto the surface of a material to be analyzed, a diaphragm-aperture for passing ions backscattered from the surface only at a particular angle, and a detector for measuring the kinetic energy of the passed backscattered ions.
Robinson has described in U.S. Pat. No. 4,217,495 an SEM having a particular scintillator structure for improving the collection of backscattered electrons.